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Is it possible to read the HTML body before the DOM-ready event?

I need to read the HTML body content (either as DOM or as a raw string) as soon as possible to optimize some loading of further scripts (load script A if content is X, or B if content is Y). Is it possible to avoid waiting for all sync scripts to load (which is a condition for DOM-ready) in order to read the body?
Note: changing scripts to be async is not an option.
Update: I do not control the server, so the optimization cannot be done there.

Perhaps look at the state changes by subscribing to that. Inline comments for each.
// self envoking
(function() {
console.log("self envoking in script");
}());
console.log(document.readyState); // logs "loading" first
window.onload = function() {
console.log('onload'); // I fire later
}
document.addEventListener("DOMContentLoaded", function() {
console.log('DOMContentLoaded');
});
document.addEventListener('readystatechange', function() {
if (document.readyState == "loading") {
//document is loading, does not fire here since no change from "loading" to "loading"
}
if (document.readyState == "interactive") {
//document fully read. fires before DOMContentLoaded
}
if (document.readyState == "complete") {
//document fully read and all resources (like images) loaded. fires after DOMContentLoaded
}
console.log(document.readyState)
});
<script id="myscript">
// self envoking
(function() {
console.log("self envoking with id");
window.customevent = new Event('customload');
// Listen for the event.
document.addEventListener('customload', function(e) {
console.log("customload fired");
}, false);
// another way
var scriptNode = document.createElement('script');
scriptNode.innerHTML = 'console.log("happy happy day inject");';
document.head.appendChild(scriptNode);
}());
</script>
<script id="another">
var thisScriptElement = document.getElementById("another");
// self envoking
(function() {
console.log("Another self envoking with id");
// Dispatch the event.
document.dispatchEvent(customevent);
}());
</script>

External JavaScript with jQuery dependence being loaded before jQuery

Externally loading a script, but my script was placed by the client above jQuery (which is a requirement), as such, my script does not work.
I am trying to make my code wait until jQuery has loaded before executing, but I am having difficulty with nested functions within my code; specifically $(x).hover, or $(x).click etc.
I can separate my functions without much trouble, which include jQuery selectors (but they won't be called unless 'x y or z' is done (i.e. until after jQuery is loaded).
I don't know how to have the hover, click etc implemented as they don't work within my $(document).ready(function(){... which is located within the onload yourFunctionName described below - with thanks to user #chaos
Link to onload hook: https://stackoverflow.com/a/807997/1173155
and a quote of the above link:
if(window.attachEvent) {
window.attachEvent('onload', yourFunctionName);
} else {
if(window.onload) {
var curronload = window.onload;
var newonload = function() {
curronload();
yourFunctionName();
};
window.onload = newonload;
} else {
window.onload = yourFunctionName;
}
}
Thanks in advance for any help with this.
I have also looked into a loop that checks if jQuery is activated before continueing, but did not implement it as I found that JavaScript does not have a sufficient sleep method that sleeps that specific script.

Solution:
if(typeof jQuery === "undefined"){
if(window.attachEvent) {
window.attachEvent('onload', myLoadFunction);
} else {
if(window.onload) {
var curronload = window.onload;
var newonload = function() {
curronload();
myLoadFunction();
};
window.onload = newonload;
} else {
window.onload = myLoadFunction;
}
}
}
else {
myLoadFunction();
}

How to trigger a custom javascript event in IE8?

I'm trying to fire a custom event on IE8 and fiddling a solution together from here and here. But I cannot get it to work...
I'm using jquery mobile with requireJS and google analytics. So I'm tracking the JQM pageshow event. However since requireJS loads scripts async, my binding to pageshow needs to be made in a javascript "wrapper", otherwise it will produce an error, because neither jquery nor jquery mobile will have been loaded by the time the snippet is parsed.
So I'm doing including this at the end of every page:
if (document.addEventListener) {
document.addEventListener("jqmReady",function(){trigAnalytics("jqmReady");alert("FF detected")},false);
} else if ( document.attachEvent ) {
document.attachEvent("jqmReady", function(){trigAnalytics("jqmReady");alert("IE detected")});
}
And when detected, I'm firing my analytics snippet with the pageshow binding:
var trigAnalytics = function( trigger ){
$(document).on('pageshow','div:jqmData(role="page").basePage', function (event, ui) {
var url = location.href;
try {
hash = location.hash;
if (hash && hash.length > 1) {
_gaq.push(['_trackPageview', hash.substr(1)]);
_gaq.push(['_setCustomVar', 1, 'id_external', ########, 1 ]);
} else {
_gaq.push(['_trackPageview', url]);
_gaq.push(['_setCustomVar', 1, 'id_external', ########, , 1 ]);
_gaq.push(['b._trackPageview', url]);
}
} catch(err) { }
});
if (typeof _gaq !== "undefined" && _gaq !== null) {
$(document).ajaxSend(function(event, xhr, settings){
_gaq.push(['_trackPageview', settings.url]);
_gaq.push(['b._trackPageview', settings.url]);
});
}
};
So to kick of the event chain, I need to trigger jqmReady when JQM is ready. JQM uses their mobileinit event to indicate just that. So inside my application controller init, I'm binding to it like so:
$(document).bind("mobileinit", function () {
// non-IE OK
if (document.createEvent) {
evt = document.createEvent("Event");
evt.initEvent("jqmReady", true, true);
document.dispatchEvent(evt);
} else if (document.createEventObject) {
// MSIE (NOT WORKING)
document.documentElement.evt = 0; // an expando property
document.documentElement.attachEvent("jqmReady", function () {
document.documentElement.evt = document.documentElement.evt + 1;
});
}
});
I have tried just triggering $(window).trigger('jqmReady'), because when mobileinit triggers, jquery is available. However it seems events created in addEventListener can not be triggered like this, so I need a javascript-only solution to trigger a custom event in IE.
Question:
Can someone give me a pointer on how to trigger a javascript custom event for IE8 correctly?

Ok, I finally understand... here is how it works:
1) setting the listener for jqmReady on the page being loaded
// non-IE: just create a listener for the custom event "jqmReady"
if (document.addEventListener) {
document.addEventListener("jqmReady",function(){trigAnalytics("jqmReady");alert("FF detected")},false);
// IE8
} else if ( document.attachEvent ) {
// create a custom property name jqmReady and set it to 0
document.documentElement.jqmReady = 0;
// since IE8 does not allow to listen to custom events,
// just listen to onpropertychange
document.documentElement.attachEvent("onpropertychange", function(event) {
// if the property changed is the custom jqmReady property
if (event.propertyName == "jqmReady") {
trigAnalytics("jqmReady");
alert("gotcha")
// remove listener, since it's only used once
document.documentElement.detachEvent("onpropertychange", arguments.callee);
}
});
}
So on IE8 I'm not listening for custom jqmReady. Instead I listen for onpropertychange for my custom property jqmReady
2) Then on mobileinit I'm triggering like this:
// non-IE
if (document.createEvent) {
evt = document.createEvent("Event");
evt.initEvent("jqmReady", true, true);
document.dispatchEvent(evt);
} else if (document.createEventObject) { // MSIE
// just change the property
// this will trigger onpropertychange
document.documentElement.jqmReady++;
};
Nice idea (credit to http://dean.edwards.name/weblog/2009/03/callbacks-vs-events/), maybe someone else can find a use for it.

For anyone else interested, I've wrapped up this code into a static javascript object
function Event () {
}
Event.listen = function (eventName, callback) {
if(document.addEventListener) {
document.addEventListener(eventName, callback, false);
} else {
document.documentElement.attachEvent('onpropertychange', function (e) {
if(e.propertyName == eventName) {
callback();
}
});
}
}
Event.trigger = function (eventName) {
if(document.createEvent) {
var event = document.createEvent('Event');
event.initEvent(eventName, true, true);
document.dispatchEvent(event);
} else {
document.documentElement[eventName]++;
}
}
usage:
Event.listen('myevent', function () {
alert('myevent triggered!');
});
Event.trigger('myevent');
Demo:
http://jsfiddle.net/c5CuF/

What is the non-jQuery equivalent of '$(document).ready()'?

What is the non-jQuery equivalent of $(document).ready()?

This works perfectly, from ECMA. The snippet is all you need, but if you want to dig more and explore other options check this detailed explanation.
document.addEventListener("DOMContentLoaded", function() {
// code...
});
The window.onload doesn't equal to JQuery $(document).ready because $(document).ready waits only to the DOM tree while window.onload check all elements including external assets and images.
EDIT: Added IE8 and older equivalent, thanks to Jan Derk's observation. You may read the source of this code on MDN:
// alternative to DOMContentLoaded
document.onreadystatechange = function () {
if (document.readyState == "interactive") {
// Initialize your application or run some code.
}
}
There are other options apart from "interactive". See the MDN docs for details.

Now that it's 2018 here's a quick and simple method.
This will add an event listener, but if it already fired we'll check that the dom is in a ready state or that it's complete. This can fire before or after sub-resources have finished loading (images, stylesheets, frames, etc).
function domReady(fn) {
// If we're early to the party
document.addEventListener("DOMContentLoaded", fn);
// If late; I mean on time.
if (document.readyState === "interactive" || document.readyState === "complete" ) {
fn();
}
}
domReady(() => console.log("DOM is ready, come and get it!"));
Additional Readings
https://developer.mozilla.org/en-US/docs/Web/Events/DOMContentLoaded
Checking whether loading is already complete
https://developer.mozilla.org/en-US/docs/Web/API/Document/readyState
Update
Here's some quick utility helpers using standard ES6 Import & Export I wrote that include TypeScript as well. Maybe I can get around to making these a quick library that can be installed into projects as a dependency.
JavaScript
export const domReady = (callBack) => {
if (document.readyState === "loading") {
document.addEventListener('DOMContentLoaded', callBack);
}
else {
callBack();
}
}
export const windowReady = (callBack) => {
if (document.readyState === 'complete') {
callBack();
}
else {
window.addEventListener('load', callBack);
}
}
TypeScript
export const domReady = (callBack: () => void) => {
if (document.readyState === "loading") {
document.addEventListener('DOMContentLoaded', callBack);
}
else {
callBack();
}
}
export const windowReady = (callBack: () => void) => {
if (document.readyState === 'complete') {
callBack();
}
else {
window.addEventListener('load', callBack);
}
}
Promises
export const domReady = new Promise(resolve => {
if (document.readyState === "loading") {
document.addEventListener('DOMContentLoaded', resolve);
}
else {
resolve();
}
});
export const windowReady = new Promise(resolve => {
if (document.readyState === 'complete') {
resolve();
}
else {
window.addEventListener('load', resolve);
}
});

A little thing I put together
domready.js
(function(exports, d) {
function domReady(fn, context) {
function onReady(event) {
d.removeEventListener("DOMContentLoaded", onReady);
fn.call(context || exports, event);
}
function onReadyIe(event) {
if (d.readyState === "complete") {
d.detachEvent("onreadystatechange", onReadyIe);
fn.call(context || exports, event);
}
}
d.addEventListener && d.addEventListener("DOMContentLoaded", onReady) ||
d.attachEvent && d.attachEvent("onreadystatechange", onReadyIe);
}
exports.domReady = domReady;
})(window, document);
How to use it
<script src="domready.js"></script>
<script>
domReady(function(event) {
alert("dom is ready!");
});
</script>
You can also change the context in which the callback runs by passing a second argument
function init(event) {
alert("check the console");
this.log(event);
}
domReady(init, console);

There is a standards based replacement
DOMContentLoaded that is supported by over 90%+ of browsers, but not
IE8 (So below code use by JQuery for browser support)
document.addEventListener("DOMContentLoaded", function(event) {
//do work
});
jQuery's native function is much more complicated than just window.onload, as depicted below.
function bindReady(){
if ( readyBound ) return;
readyBound = true;
// Mozilla, Opera and webkit nightlies currently support this event
if ( document.addEventListener ) {
// Use the handy event callback
document.addEventListener( "DOMContentLoaded", function(){
document.removeEventListener( "DOMContentLoaded", arguments.callee, false );
jQuery.ready();
}, false );
// If IE event model is used
} else if ( document.attachEvent ) {
// ensure firing before onload,
// maybe late but safe also for iframes
document.attachEvent("onreadystatechange", function(){
if ( document.readyState === "complete" ) {
document.detachEvent( "onreadystatechange", arguments.callee );
jQuery.ready();
}
});
// If IE and not an iframe
// continually check to see if the document is ready
if ( document.documentElement.doScroll && window == window.top ) (function(){
if ( jQuery.isReady ) return;
try {
// If IE is used, use the trick by Diego Perini
// http://javascript.nwbox.com/IEContentLoaded/
document.documentElement.doScroll("left");
} catch( error ) {
setTimeout( arguments.callee, 0 );
return;
}
// and execute any waiting functions
jQuery.ready();
})();
}
// A fallback to window.onload, that will always work
jQuery.event.add( window, "load", jQuery.ready );
}

According to http://youmightnotneedjquery.com/#ready a nice replacement that still works with IE8 is
function ready(fn) {
if (document.readyState != 'loading') {
fn();
} else if (document.addEventListener) {
document.addEventListener('DOMContentLoaded', fn);
} else {
document.attachEvent('onreadystatechange', function() {
if (document.readyState != 'loading')
fn();
});
}
}
// test
window.ready(function() {
alert('it works');
});
improvements: Personally I would also check if the type of fn is a function.
And as #elliottregan suggested remove the event listener after use.
function ready(fn) {
if (typeof fn !== 'function') {
throw new Error('Argument passed to ready should be a function');
}
if (document.readyState != 'loading') {
fn();
} else if (document.addEventListener) {
document.addEventListener('DOMContentLoaded', fn, {
once: true // A boolean value indicating that the listener should be invoked at most once after being added. If true, the listener would be automatically removed when invoked.
});
} else {
document.attachEvent('onreadystatechange', function() {
if (document.readyState != 'loading')
fn();
});
}
}
// tests
try {
window.ready(5);
} catch (ex) {
console.log(ex.message);
}
window.ready(function() {
alert('it works');
});
The reason I answer this question late is because I was searching for this answer but could not find it here. And I think this is the best solution.

This does not answer the question nor does it show any non-jQuery code. See # sospedra's answer below.
The nice thing about $(document).ready() is that it fires before window.onload. The load function waits until everything is loaded, including external assets and images. $(document).ready, however, fires when the DOM tree is complete and can be manipulated. If you want to acheive DOM ready, without jQuery, you might check into this library. Someone extracted just the ready part from jQuery. Its nice and small and you might find it useful:
domready at Google Code

The easiest way in recent browsers would be to use the appropriate GlobalEventHandlers, onDOMContentLoaded, onload, onloadeddata (...)
onDOMContentLoaded = (function(){ console.log("DOM ready!") })()
onload = (function(){ console.log("Page fully loaded!") })()
onloadeddata = (function(){ console.log("Data loaded!") })()
The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading. A very different event load should be used only to detect a fully-loaded page. It is an incredibly popular mistake to use load where DOMContentLoaded would be much more appropriate, so be cautious.
https://developer.mozilla.org/en-US/docs/Web/Events/DOMContentLoaded
The function used is an IIFE, very useful on this case, as it trigger itself when ready:
https://en.wikipedia.org/wiki/Immediately-invoked_function_expression
It is obviously more appropriate to place it at the end of any scripts.
In ES6, we can also write it as an arrow function:
onload = (() => { console.log("ES6 page fully loaded!") })()
The best is to use the DOM elements, we can wait for any variable to be ready, that trigger an arrowed IIFE.
The behavior will be the same, but with less memory impact.
footer = (() => { console.log("Footer loaded!") })()
<div id="footer">

In plain vanilla JavaScript, with no libraries? It's an error. $ is simply an identifier, and is undefined unless you define it.
jQuery defines $ as it's own "everything object" (also known as jQuery so you can use it without conflicting with other libraries). If you're not using jQuery (or some other library that defines it), then $ will not be defined.
Or are you asking what the equivalent is in plain JavaScript? In that case, you probably want window.onload, which isn't exactly equivalent, but is the quickest and easiest way to get close to the same effect in vanilla JavaScript.

The body onLoad could be an alternative too:
<html>
<head><title>Body onLoad Exmaple</title>
<script type="text/javascript">
function window_onload() {
//do something
}
</script>
</head>
<body onLoad="window_onload()">
</body>
</html>

$(document).ready equivalent without jQuery

I have a script that uses $(document).ready, but it doesn't use anything else from jQuery. I'd like to lighten it up by removing the jQuery dependency.
How can I implement my own $(document).ready functionality without using jQuery? I know that using window.onload will not be the same, as window.onload fires after all images, frames, etc. have been loaded.

There is a standards based replacement,DOMContentLoaded that is supported by over 99% of browsers, though not IE8:
document.addEventListener("DOMContentLoaded", function(event) {
//do work
});
jQuery's native function is much more complicated than just window.onload, as depicted below.
function bindReady(){
if ( readyBound ) return;
readyBound = true;
// Mozilla, Opera and webkit nightlies currently support this event
if ( document.addEventListener ) {
// Use the handy event callback
document.addEventListener( "DOMContentLoaded", function(){
document.removeEventListener( "DOMContentLoaded", arguments.callee, false );
jQuery.ready();
}, false );
// If IE event model is used
} else if ( document.attachEvent ) {
// ensure firing before onload,
// maybe late but safe also for iframes
document.attachEvent("onreadystatechange", function(){
if ( document.readyState === "complete" ) {
document.detachEvent( "onreadystatechange", arguments.callee );
jQuery.ready();
}
});
// If IE and not an iframe
// continually check to see if the document is ready
if ( document.documentElement.doScroll && window == window.top ) (function(){
if ( jQuery.isReady ) return;
try {
// If IE is used, use the trick by Diego Perini
// http://javascript.nwbox.com/IEContentLoaded/
document.documentElement.doScroll("left");
} catch( error ) {
setTimeout( arguments.callee, 0 );
return;
}
// and execute any waiting functions
jQuery.ready();
})();
}
// A fallback to window.onload, that will always work
jQuery.event.add( window, "load", jQuery.ready );
}

Edit:
2023 update, use this:
function ready(fn) {
if (document.readyState !== 'loading') {
fn();
return;
}
document.addEventListener('DOMContentLoaded', fn);
}
From: https://youmightnotneedjquery.com/
Here is a viable replacement for jQuery ready
function ready(callback){
// in case the document is already rendered
if (document.readyState!='loading') callback();
// modern browsers
else if (document.addEventListener) document.addEventListener('DOMContentLoaded', callback);
// IE <= 8
else document.attachEvent('onreadystatechange', function(){
if (document.readyState=='complete') callback();
});
}
ready(function(){
// do something
});
Taken from
https://plainjs.com/javascript/events/running-code-when-the-document-is-ready-15/
Another good domReady function here taken from https://stackoverflow.com/a/9899701/175071
As the accepted answer was very far from complete, I stitched together a "ready" function like jQuery.ready() based on jQuery 1.6.2 source:
var ready = (function(){
var readyList,
DOMContentLoaded,
class2type = {};
class2type["[object Boolean]"] = "boolean";
class2type["[object Number]"] = "number";
class2type["[object String]"] = "string";
class2type["[object Function]"] = "function";
class2type["[object Array]"] = "array";
class2type["[object Date]"] = "date";
class2type["[object RegExp]"] = "regexp";
class2type["[object Object]"] = "object";
var ReadyObj = {
// Is the DOM ready to be used? Set to true once it occurs.
isReady: false,
// A counter to track how many items to wait for before
// the ready event fires. See #6781
readyWait: 1,
// Hold (or release) the ready event
holdReady: function( hold ) {
if ( hold ) {
ReadyObj.readyWait++;
} else {
ReadyObj.ready( true );
}
},
// Handle when the DOM is ready
ready: function( wait ) {
// Either a released hold or an DOMready/load event and not yet ready
if ( (wait === true && !--ReadyObj.readyWait) || (wait !== true && !ReadyObj.isReady) ) {
// Make sure body exists, at least, in case IE gets a little overzealous (ticket #5443).
if ( !document.body ) {
return setTimeout( ReadyObj.ready, 1 );
}
// Remember that the DOM is ready
ReadyObj.isReady = true;
// If a normal DOM Ready event fired, decrement, and wait if need be
if ( wait !== true && --ReadyObj.readyWait > 0 ) {
return;
}
// If there are functions bound, to execute
readyList.resolveWith( document, [ ReadyObj ] );
// Trigger any bound ready events
//if ( ReadyObj.fn.trigger ) {
// ReadyObj( document ).trigger( "ready" ).unbind( "ready" );
//}
}
},
bindReady: function() {
if ( readyList ) {
return;
}
readyList = ReadyObj._Deferred();
// Catch cases where $(document).ready() is called after the
// browser event has already occurred.
if ( document.readyState === "complete" ) {
// Handle it asynchronously to allow scripts the opportunity to delay ready
return setTimeout( ReadyObj.ready, 1 );
}
// Mozilla, Opera and webkit nightlies currently support this event
if ( document.addEventListener ) {
// Use the handy event callback
document.addEventListener( "DOMContentLoaded", DOMContentLoaded, false );
// A fallback to window.onload, that will always work
window.addEventListener( "load", ReadyObj.ready, false );
// If IE event model is used
} else if ( document.attachEvent ) {
// ensure firing before onload,
// maybe late but safe also for iframes
document.attachEvent( "onreadystatechange", DOMContentLoaded );
// A fallback to window.onload, that will always work
window.attachEvent( "onload", ReadyObj.ready );
// If IE and not a frame
// continually check to see if the document is ready
var toplevel = false;
try {
toplevel = window.frameElement == null;
} catch(e) {}
if ( document.documentElement.doScroll && toplevel ) {
doScrollCheck();
}
}
},
_Deferred: function() {
var // callbacks list
callbacks = [],
// stored [ context , args ]
fired,
// to avoid firing when already doing so
firing,
// flag to know if the deferred has been cancelled
cancelled,
// the deferred itself
deferred = {
// done( f1, f2, ...)
done: function() {
if ( !cancelled ) {
var args = arguments,
i,
length,
elem,
type,
_fired;
if ( fired ) {
_fired = fired;
fired = 0;
}
for ( i = 0, length = args.length; i < length; i++ ) {
elem = args[ i ];
type = ReadyObj.type( elem );
if ( type === "array" ) {
deferred.done.apply( deferred, elem );
} else if ( type === "function" ) {
callbacks.push( elem );
}
}
if ( _fired ) {
deferred.resolveWith( _fired[ 0 ], _fired[ 1 ] );
}
}
return this;
},
// resolve with given context and args
resolveWith: function( context, args ) {
if ( !cancelled && !fired && !firing ) {
// make sure args are available (#8421)
args = args || [];
firing = 1;
try {
while( callbacks[ 0 ] ) {
callbacks.shift().apply( context, args );//shifts a callback, and applies it to document
}
}
finally {
fired = [ context, args ];
firing = 0;
}
}
return this;
},
// resolve with this as context and given arguments
resolve: function() {
deferred.resolveWith( this, arguments );
return this;
},
// Has this deferred been resolved?
isResolved: function() {
return !!( firing || fired );
},
// Cancel
cancel: function() {
cancelled = 1;
callbacks = [];
return this;
}
};
return deferred;
},
type: function( obj ) {
return obj == null ?
String( obj ) :
class2type[ Object.prototype.toString.call(obj) ] || "object";
}
}
// The DOM ready check for Internet Explorer
function doScrollCheck() {
if ( ReadyObj.isReady ) {
return;
}
try {
// If IE is used, use the trick by Diego Perini
// http://javascript.nwbox.com/IEContentLoaded/
document.documentElement.doScroll("left");
} catch(e) {
setTimeout( doScrollCheck, 1 );
return;
}
// and execute any waiting functions
ReadyObj.ready();
}
// Cleanup functions for the document ready method
if ( document.addEventListener ) {
DOMContentLoaded = function() {
document.removeEventListener( "DOMContentLoaded", DOMContentLoaded, false );
ReadyObj.ready();
};
} else if ( document.attachEvent ) {
DOMContentLoaded = function() {
// Make sure body exists, at least, in case IE gets a little overzealous (ticket #5443).
if ( document.readyState === "complete" ) {
document.detachEvent( "onreadystatechange", DOMContentLoaded );
ReadyObj.ready();
}
};
}
function ready( fn ) {
// Attach the listeners
ReadyObj.bindReady();
var type = ReadyObj.type( fn );
// Add the callback
readyList.done( fn );//readyList is result of _Deferred()
}
return ready;
})();
How to use:
<script>
ready(function(){
alert('It works!');
});
ready(function(){
alert('Also works!');
});
</script>
I am not sure how functional this code is, but it worked fine with my superficial tests. This took quite a while, so I hope you and others can benefit from it.
PS.: I suggest compiling it.
Or you can use http://dustindiaz.com/smallest-domready-ever:
function r(f){/in/.test(document.readyState)?setTimeout(r,9,f):f()}
r(function(){/*code to run*/});
or the native function if you only need to support the new browsers (Unlike jQuery ready, this won't run if you add this after the page has loaded)
document.addEventListener('DOMContentLoaded',function(){/*fun code to run*/})

Three options:
If script is the last tag of the body, the DOM would be ready before script tag executes
When the DOM is ready, "readyState" will change to "complete"
Put everything under 'DOMContentLoaded' event listener
onreadystatechange
document.onreadystatechange = function () {
if (document.readyState == "complete") {
// document is ready. Do your stuff here
}
}
Source: MDN
DOMContentLoaded
document.addEventListener('DOMContentLoaded', function() {
console.log('document is ready. I can sleep now');
});
Concerned about stone age browsers:
Go to the jQuery source code and use the ready function. In that case you are not parsing+executing the whole library you're are doing only a very small part of it.

Place your <script>/*JavaScript code*/</script> right before the closing </body> tag.
Admittedly, this might not suit everyone's purposes since it requires changing the HTML file rather than just doing something in the JavaScript file a la document.ready, but still...

Poor man's solution:
var checkLoad = function() {
document.readyState !== "complete" ? setTimeout(checkLoad, 11) : alert("loaded!");
};
checkLoad();
View Fiddle
Added this one, a bit better I guess, own scope, and non recursive
(function(){
var tId = setInterval(function() {
if (document.readyState == "complete") onComplete()
}, 11);
function onComplete(){
clearInterval(tId);
alert("loaded!");
};
})()
View Fiddle

I use this:
document.addEventListener("DOMContentLoaded", function(event) {
//Do work
});
Note: This probably only works with newer browsers, especially these: http://caniuse.com/#feat=domcontentloaded

It is year 2020 and <script> tag has defer attribute.
for example:
<script src="demo_defer.js" defer></script>
it specifies that the script is executed when the page has finished parsing.
https://www.w3schools.com/tags/att_script_defer.asp

2022 version
In 2022, all you need to do is put the defer attribute on your script, and load it in the head!
Reference: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#attr-defer
<!doctype html>
<html>
<head>
<script src="/script.js" defer></script>
</head>
<body>
<p>In 2022, all you need to do is put the defer attribute on your script, and load it in the head!</p>
</body>
</html>

This question was asked quite a long time ago. For anyone just seeing this question, there is now a site called "you might not need jquery" which breaks down - by level of IE support required - all the functionality of jquery and provides some alternative, smaller libraries.
IE8 document ready script according to you might not need jquery
function ready(fn) {
if (document.readyState != 'loading')
fn();
else if (document.addEventListener)
document.addEventListener('DOMContentLoaded', fn);
else
document.attachEvent('onreadystatechange', function() {
if (document.readyState != 'loading')
fn();
});
}

Really, if you care about Internet Explorer 9+ only, this code would be enough to replace jQuery.ready:
document.addEventListener("DOMContentLoaded", callback);
If you worry about Internet Explorer 6 and some really strange and rare browsers, this will work:
domReady: function (callback) {
// Mozilla, Opera and WebKit
if (document.addEventListener) {
document.addEventListener("DOMContentLoaded", callback, false);
// If Internet Explorer, the event model is used
} else if (document.attachEvent) {
document.attachEvent("onreadystatechange", function() {
if (document.readyState === "complete" ) {
callback();
}
});
// A fallback to window.onload, that will always work
} else {
var oldOnload = window.onload;
window.onload = function () {
oldOnload && oldOnload();
callback();
}
}
},

Cross-browser (old browsers too) and a simple solution:
var docLoaded = setInterval(function () {
if(document.readyState !== "complete") return;
clearInterval(docLoaded);
/*
Your code goes here i.e. init()
*/
}, 30);
Showing alert in jsfiddle

I was recently using this for a mobile site. This is John Resig's simplified version from "Pro JavaScript Techniques". It depends on addEvent.
var ready = ( function () {
function ready( f ) {
if( ready.done ) return f();
if( ready.timer ) {
ready.ready.push(f);
} else {
addEvent( window, "load", isDOMReady );
ready.ready = [ f ];
ready.timer = setInterval(isDOMReady, 13);
}
};
function isDOMReady() {
if( ready.done ) return false;
if( document && document.getElementsByTagName && document.getElementById && document.body ) {
clearInterval( ready.timer );
ready.timer = null;
for( var i = 0; i < ready.ready.length; i++ ) {
ready.ready[i]();
}
ready.ready = null;
ready.done = true;
}
}
return ready;
})();

The jQuery answer was pretty useful to me. With a little refactory it fitted my needs well.
I hope it helps anybody else.
function onReady ( callback ){
var addListener = document.addEventListener || document.attachEvent,
removeListener = document.removeEventListener || document.detachEvent
eventName = document.addEventListener ? "DOMContentLoaded" : "onreadystatechange"
addListener.call(document, eventName, function(){
removeListener( eventName, arguments.callee, false )
callback()
}, false )
}

Here is the smallest code snippet to test DOM ready which works across all browsers (even IE 8):
r(function(){
alert('DOM Ready!');
});
function r(f){/in/.test(document.readyState)?setTimeout('r('+f+')',9):f()}
See this answer.

Just add this to the bottom of your HTML page...
<script>
Your_Function();
</script>
Because, HTML documents are parsed by top-bottom.

Most minimal and 100% working
I have picked the answer from PlainJS and it's working fine for me. It extends DOMContentLoaded so that it can be accepted at all the browsers.
This function is the equivalent of jQuery's $(document).ready() method:
document.addEventListener('DOMContentLoaded', function(){
// do something
});
However, in contrast to jQuery, this code will only run properly in modern browsers (IE > 8) and it won't in case the document is already rendered at the time this script gets inserted (e.g. via Ajax). Therefore, we need to extend this a little bit:
function run() {
// do something
}
// in case the document is already rendered
if (document.readyState!='loading') run();
// modern browsers
else if (document.addEventListener)
document.addEventListener('DOMContentLoaded', run);
// IE <= 8
else document.attachEvent('onreadystatechange', function(){
if (document.readyState=='complete') run();
});
This covers basically all possibilities and is a viable replacement for the jQuery helper.

Simplest way using pure JavaScript. Without jQuery:
document.addEventListener("DOMContentLoaded", function(event) {
// Your code to run since DOM is loaded and ready
});

It is worth looking in Rock Solid addEvent() and http://www.braksator.com/how-to-make-your-own-jquery.
Here is the code in case the site goes down
function addEvent(obj, type, fn) {
if (obj.addEventListener) {
obj.addEventListener(type, fn, false);
EventCache.add(obj, type, fn);
}
else if (obj.attachEvent) {
obj["e"+type+fn] = fn;
obj[type+fn] = function() { obj["e"+type+fn]( window.event ); }
obj.attachEvent( "on"+type, obj[type+fn] );
EventCache.add(obj, type, fn);
}
else {
obj["on"+type] = obj["e"+type+fn];
}
}
var EventCache = function(){
var listEvents = [];
return {
listEvents : listEvents,
add : function(node, sEventName, fHandler){
listEvents.push(arguments);
},
flush : function(){
var i, item;
for(i = listEvents.length - 1; i >= 0; i = i - 1){
item = listEvents[i];
if(item[0].removeEventListener){
item[0].removeEventListener(item[1], item[2], item[3]);
};
if(item[1].substring(0, 2) != "on"){
item[1] = "on" + item[1];
};
if(item[0].detachEvent){
item[0].detachEvent(item[1], item[2]);
};
item[0][item[1]] = null;
};
}
};
}();
// Usage
addEvent(window, 'unload', EventCache.flush);
addEvent(window, 'load', function(){alert("I'm ready");});

It's always good to use JavaScript equivalents as compared to jQuery. One reason is one fewer library to depend on and they are much faster than the jQuery equivalents.
One fantastic reference for jQuery equivalents is http://youmightnotneedjquery.com/.
As far as your question is concerned, I took the below code from the above link :)
Only caveat is it only works with Internet Explorer 9 and later.
function ready(fn) {
if (document.readyState != 'loading') {
fn();
}
else {
document.addEventListener('DOMContentLoaded', fn);
}
}

This cross-browser code will call a function once the DOM is ready:
var domReady=function(func){
var scriptText='('+func+')();';
var scriptElement=document.createElement('script');
scriptElement.innerText=scriptText;
document.body.appendChild(scriptElement);
};
Here's how it works:
The first line of domReady calls the toString method of the function to get a string representation of the function you pass in and wraps it in an expression that immediately calls the function.
The rest of domReady creates a script element with the expression and appends it to the body of the document.
The browser runs script tags appended to body after the DOM is ready.
For example, if you do this: domReady(function(){alert();});, the following will appended to the body element:
<script>(function (){alert();})();</script>
Note that this works only for user-defined functions. The following won't work: domReady(alert);

How about this solution?
// other onload attached earlier
window.onload=function() {
alert('test');
};
tmpPreviousFunction=window.onload ? window.onload : null;
// our onload function
window.onload=function() {
alert('another message');
// execute previous one
if (tmpPreviousFunction) tmpPreviousFunction();
};

We found a quick-and-dirty cross browser implementation of ours that may do the trick for most simple cases with a minimal implementation:
window.onReady = function onReady(fn){
document.body ? fn() : setTimeout(function(){ onReady(fn);},50);
};

I simply use:
setTimeout(function(){
//reference/manipulate DOM here
});
And unlike document.addEventListener("DOMContentLoaded" //etc as in the very top answer, it works as far back as IE9 -- http://caniuse.com/#search=DOMContentLoaded only indicates as recently as IE11.
Interestingly I stumbled upon this setTimeout solution in 2009: Is checking for the readiness of the DOM overkill?, which probably could have been worded slightly better, as I meant "is it overkill to use various frameworks' more complicated approaches to check for the readiness of the DOM".
My best explanation for why this technique works is that, when the script with such a setTimeout has been reached, the DOM is in the middle of being parsed, so execution of the code within the setTimeout gets deferred until that operation is finished.

Comparison
Here (in below snippet) is comparison of chosen available browser "built-in" methods and their execution sequence. Remarks
the document.onload (X) is not supported by any modern browser (event is never fired)
if you use <body onload="bodyOnLoad()"> (F) and at the same time window.onload (E) then only first one will be executed (because it override second one)
event handler given in <body onload="..."> (F) is wrapped by additional onload function
document.onreadystatechange (D) not override document .addEventListener('readystatechange'...) (C) probably cecasue onXYZevent-like methods are independent than addEventListener queues (which allows add multiple listeners). Probably nothing happens between execution this two handlers.
all scripts write their timestamp in console - but scripts which also have access to div write their timestamps also in body (click "Full Page" link after script execution to see it).
solutions readystatechange (C,D) are executed multiple times by browser but for different document states:
loading - the document is loading (no fired in snippet)
interactive - the document is parsed, fired before DOMContentLoaded
complete - the document and resources are loaded, fired before body/window onload
<html>
<head>
<script>
// solution A
console.log(`[timestamp: ${Date.now()}] A: Head script`) ;
// solution B
document.addEventListener("DOMContentLoaded", () => {
print(`[timestamp: ${Date.now()}] B: DOMContentLoaded`);
});
// solution C
document.addEventListener('readystatechange', () => {
print(`[timestamp: ${Date.now()}] C: ReadyState: ${document.readyState}`);
});
// solution D
document.onreadystatechange = s=> {print(`[timestamp: ${Date.now()}] D: document.onreadystatechange ReadyState: ${document.readyState}`)};
// solution E (never executed)
window.onload = () => {
print(`E: <body onload="..."> override this handler`);
};
// solution F
function bodyOnLoad() {
print(`[timestamp: ${Date.now()}] F: <body onload='...'>`);
infoAboutOnLoad(); // additional info
}
// solution X
document.onload = () => {print(`document.onload is never fired`)};
// HELPERS
function print(txt) {
console.log(txt);
if(mydiv) mydiv.innerHTML += txt.replace('<','<').replace('>','>') + '<br>';
}
function infoAboutOnLoad() {
console.log("window.onload (after override):", (''+document.body.onload).replace(/\s+/g,' '));
console.log(`body.onload==window.onload --> ${document.body.onload==window.onload}`);
}
console.log("window.onload (before override):", (''+document.body.onload).replace(/\s+/g,' '));
</script>
</head>
<body onload="bodyOnLoad()">
<div id="mydiv"></div>
<!-- this script must te at the bottom of <body> -->
<script>
// solution G
print(`[timestamp: ${Date.now()}] G: <body> bottom script`);
</script>
</body>
</html>

Nowadays you should use modules. Put your code into the default function of a module and import the function into a script element.
client.js:
export default function ()
{
alert ("test");
}
index.html:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>test</title>
</head>
<body>
<script type="module">
import main from './client.js';
main ();
</script>
</body>
</html>

Here's what I use, it's fast and covers all bases I think; works for everything except IE<9.
(() => { function fn() {
// "On document ready" commands:
console.log(document.readyState);
};
if (document.readyState != 'loading') {fn()}
else {document.addEventListener('DOMContentLoaded', fn)}
})();
This seems to catch all cases:
fires immediately if the DOM is already ready (if the DOM is not "loading", but either "interactive" or "complete")
if the DOM is still loading, it sets up an event listener for when the DOM
is available (interactive).
The DOMContentLoaded event is available in IE9 and everything else, so I personally think it's OK to use this. Rewrite the arrow function declaration to a regular anonymous function if you're not transpiling your code from ES2015 to ES5.
If you want to wait until all assets are loaded, all images displayed etc then use window.onload instead.

If you don't have to support very old browsers, here is a way to do it even when your external script is loaded with async attribute:
HTMLDocument.prototype.ready = new Promise(function(resolve) {
if(document.readyState != "loading")
resolve();
else
document.addEventListener("DOMContentLoaded", function() {
resolve();
});
});
document.ready.then(function() {
console.log("document.ready");
});

For IE9+:
function ready(fn) {
if (document.readyState != 'loading'){
fn();
} else {
document.addEventListener('DOMContentLoaded', fn);
}
}

The setTimeout/setInterval solutions presented here will only work in specific circumstances.
The problem shows up especially in older Internet Explorer versions up to 8.
The variables affecting the success of these setTimeout/setInterval solutions are:
1) dynamic or static HTML
2) cached or non cached requests
3) size of the complete HTML document
4) chunked or non chunked transfer encoding
the original (native Javascript) code solving this specific issue is here:
https://github.com/dperini/ContentLoaded
http://javascript.nwbox.com/ContentLoaded (test)
this is the code from which the jQuery team have built their implementation.

If you are loading jQuery near the bottom of BODY, but are having trouble with code that writes out jQuery(<func>) or jQuery(document).ready(<func>), check out jqShim on Github.
Rather than recreate its own document ready function, it simply holds onto the functions until jQuery is available then proceeds with jQuery as expected. The point of moving jQuery to the bottom of body is to speed up page load, and you can still accomplish it by inlining the jqShim.min.js in the head of your template.
I ended up writing this code to make moving all the scripts in WordPress to the footer, and just this shim code now sits directly in the header.